Method of growing crystals



March 30 1926. 1,578,677

A. M. NICOLSON METHOD CF GROWING CRYSTALS Filed Sept. ZO. 1924 PatentedMar. 30, l1926.

UNITED STATI-:s

4PATENT OFFICE.

ALEXANDER M. NICOLSON, OF NEW YORK, N. Y., ASSIGNOR 'T0 WESTERN ELECTRICCOMPANY, INCORPORATED, OF NEW YORK, N. Y., A CORPORATION Ol' NEW YORK.

METHOD OF GROWING CRYSTALS.

Application inea september so, 1924. semi No. 740,747.

To all whom zt may concern.'

Be it known that I, ALEXANDER M. Nicol.- soN, asubject of the King ofGreat Britain, residin at INew York, in the county of'New York, tate ofNew York, have invented cer- .tain new and useful. Improvements inlMethods of Growing CrystalsVof which the following is a full, clear,concise, and exact description.

This invention relates to crystals of chemical compounds and to methodsof producing the same.

It has for an object to improve methods of producing crystals andparticularly, crystals possessing piezo-electric properties.

Another object is the provision of highly efficient methods of growingcrystals and methods which will yield uniform products in any desiredquantity.

The invention has particular reference broadly to crystals having thehour-glass structure and specifically to crystals of Rochelle salt.

An understanding of the hour-glass structure, as applied specifically toRochelle salt crystals, will be evident from the d'etailed descriptiongiven in connection with the drawings.

Fig. 1 of the drawings is an elevation showing the basal plane of acrystal of Rochelle salt having the hour-glass structure, and grown inaccordance with the method comprised in this invention.

Fig. 2 is a sectional viewin side elevation of the crystal shown inFig. 1. This view is one through the 0, a plane of the crystal.

Fig. 3 is a sectional view in end elevation of the crystal shown in Fig.1 and is a view through the 5, a plane.

Fig. 4 is a perspective view of such a crystal showing the several axesof the crystal.

Fig. 5 is an elevation of a complete crystal of Rochelle salt grownslowly and by suspension from a thread.

Fig. 6 is a top view of the crystal shown in Fig. 5. v l v It hasheretofore been known that crystals of the required size and structurefor piezoelectric appliances could be grown from seed crystals by theapplication of a temperature gradient to a saturated solution of thesalt. This method, under conditions of rapid cooling, such as atemperature drop in the mother liquor ofr from 35-38 C. to 20 C. over aperiod from 10 to 20 hours, for example, hasbee'n found to produce largecrystals having the desired hour-glass structure. Crystals of Rochellesalt may be grown in the mother liquid by suspension from a thread, byflotation on mercury 4or by being laid on a glass plate, but it has beenfound that for piezo-electric appliances it is usually desira le toemploy but half the complete crystal, shown in Figs. 5 and 6, and forthis reason the crystal is usually grown by laying a seedling, upon aglass plate or upon mercury.

Referring to Figs. 1, 2, 3 and 4 of the drawings, which indicate thegeneral appearance of crystals grown in accordance with this invention,the crystal surfaces'may be classified vinto two systems of surfaces, or

zones, which are normal to each other. One

system of zones is parallel to the principal or crystallographic axis 0,and therefore engirdles the crystal. The other system of zones comprisesthe two basal planes parallel with the a and b axes. In thisclassification of the crystal surfaces, the three axes, an b and 0, areall perpendicular to each other.

In accordance with this invention the hour-glass structure is producedby growing the crystals with the c and b axes in a horizontalposition,'in which position a crystal is represented in Fig. 4 of thedrawings. This growth forms a particular habit, becoming dominant alongthe 0 and b axes, while development along the a axis upwards issuppressed on account of the supv ersaturation and consequent gradationin density of the liguor. This upward development along the a axis maybe further suppressed by methods and for reasons pointed out in my priorPatent 1,438,965, dated December 19,1922.

The crystals are grown from perfect nuclei, possessing denite form andhaving 'a, b and 0 axes corresponding to the (1, o and c axes shown inFigs. 4, 5 and 6. In obtaining seed crystals, generally from a previouscropping, those seeds are selected in which growth along the b axis isfully developed (which happens when the seed nucleus grows with its cand b axes horizontal). These seeds are square, or nearly so, whichsignifies thatk the growth during a along the o axis is about as greatas or greater than that along the c axis. The thickness of the selectedseed crystal along its a axis is generally 4/10 of that along its baxis. This dimension is a function of the density and `head of themother liquor.

It has been noted that the crystal usually grows to the desired size, tosuch a size for instance, that it weighs about 100 grams, eriod of from5 to 10 hours and may be ta :en from the mother liquor when it hasattained such size, although growth ina be continued for days, duringthe condition of saturation, by allowing the crystal to remain in themother liquor. It appears probable that rapid growth due to this rapidcooling of the mother liquor, causes the crystal to develop internalstresses producing strain regions symmetrical with l the principal axis0. @ne cause or these stresses may be the contraction which results whenthe temperature of the mother liquor falls. In this respect, it has alsobeen noted that as Rochelle salt crystallizes, heat is given oit and itis believed that'this heat may momentarily arrest crystallization untilthe temperature again falls, when crystallization with the consequentevolution of heat, and the arrest of crystallization again results.Working upon this theory, the crystal should be developed in definitelayers corresponding to the periods during which crystallization takesplace; it has been noted in this connection that the crystal is built upin such a manner of` definite layers of Rochelle salt. It is believed,further, that this continuedcycle of events throughout thecrystallization period may cause the stress which jresults in thehour-glass structure. This composite or hour-glass structure,

'which results from this stress, is vclosely related to the surfacezones defi-ned above and to the electric poles which will now bedescribed' in relation to the composite structure.4 l

'At each end of the seed nucleusi in the crystal and along its c axisthere appears a pyramid, the base of which forms a polar terminal..These pyramids are not always pronounced in -the seed nucleus but aremore evident in the grown crystal after the latter has been dessicated,as described in my prior Patents 1,414,370, dated May 2,-

1922 and 1,438,965, dated December 19, 1922. These pyramids in thecrystal are indicated at 1 in Fig. 1, at 3 in Fig. 2, and

at 2 in Fig. 3. They consist of stratifica 'tions parallel to the b axisand normal gests that the crystal molecules throughout the piramidalregions are subject during growt to forces which turn them, in planescontaining the rincipal axis, through a right angle. This is indicatedby the fact that the crystal, after the treatment, mentioned above,whic-li tends to render more pronounced these pyramidal terminals,develops its electrical poles (plus or minus corresponding to theconventional analogous and antilogous poles) in accordance with thesystem of zones referred to above. The poles arev accordinglyorthogonal, that is, at right angles to each other. This signifies thatthe bases of .the pyramids, which occupy almost the entire area of thebasal planes of the crystal, arc electrically plus when the remainder ofthe crystal structure is minus and vice versa.

lln the production of crystals having this hour-glass structure, Abymethods heretofore employed, it has been found that not only couldsuccessive growths not be guaranteed from a specifically preparelliquor, but variations in the quality of the growth often provedunavoidable' in spite of extreme care as regards uniformity of thecrystallization operations.' These variations refer-principally to thesize and clarity of resulting growths and to the degree of spontaneousprecipitations. It was noticed for example, in practising methodsdescribed above, that the crystal of Rochelle salt was often undersized,or that a large number of nuclei were present which resulted in theproduction of a` quantity of small crystals, instead of a single largecrystal, as was desired. In this respect, applicant has discovered thatthe crystal structure, as finally produced` is determined to a largeextent not only by the conditions under which it is actually grown butalso by the manner of preparing and handling the solution which is to beused for growing crystals.

One cause of the presence of a quantity 110 of small crystals along with.one larger Rochelle salt crystal, which grew from the seedlinginserted, was thought to be the presence of impurities in the rawmaterial from which the mother liquor was produced. 115 The raw materialusually employed to form 'the saturated solution of Rochelle salt wascommercial Rochelle .salt and it was believed that this materialpossibly contained, other than pure Rochelle salt, also some 120uncombined sodium and potassium tartrates.

It has been pointed out by van Leeuwen in Zeitschrift fr Physik. Chemie1897- 223-33. that in cooling a Rochelle salt solution from above 40 C.to below 40 C. crys- ,125 tals of vboth Rochelle saltand sodium tartratewere formed and that in cooling from above 55 C. to below 55 C. thesecrystals and, in addition. potassium tartrate crystals were formed. Thissuggests a reason for the 130 possible presence of varying amounts ofsodium and potassium tartrate in commercial Rochelle salt.

Although the control of the solution up to and during the period ofcrystal growth has such a determining effect as noted above upon thecrystal structure, it is practically immaterial, according to the methodof the invention, what the previous history of the raw material has beenor whether sodium or potassium tai-trate are presentin relatively largeamounts. The process according to the invention can therefore usecommercial Rochelle salt as the raw material and this fact alone makesfor economical production since it is a. convenient land cheap form ofraw material. Y

According to the'l features of the invention, an improved method ofgrowing crystals comprises the solution of sodium potassium tartrate toform a nearly saturated solution of Rochelle salt having a historyregulated as regards temperature; then, always regulating thetemperature, growing crystals from a solution as thus prepared bystarting the hour-glass structure with rapid cooling and employingthereafter any temperature gradient which will insurev the maintainingof a super saturated mother liquor. Such a process as practiced underthe invention consists of substantially three stages, namely:

1. The process of aging.

2. The production of a stock solution.

3. The crystallization process.

Steps (l) and (2) can be combined into one more or less continuousprocess if de- Sire-d, but are given herein as two steps for clearness.l

The aging process consists substantially in allowing commercial Rochellesalt to remain in Contact with water until an equilibrium between thesodium tartrate, potassium tartrate, sodium-potassium tartrate and wateris reached, at all times keeping-the temperature below 400 C. 1t hasbeen found that such an equilibrium is reached when the density hasbecome constant with constant temperature. In allowing commercialRochelle salt to dissolve in this manner in water, it appears that theuncombined sodi# um and potassium tartrates which are believed to bepresent re-combine to form pure Rochelle salt, and in this manner thepercentage of uncombined sodium and potassium tartrates in the motherliquor is materially reduced. In accordance with the invention,the'aging process might be carried out in two steps, the first of whichmight consist in allowing commercial Rochelle salt .to dissolve inwater. while the second might consist in stirring this solution incontact with Rochelle salt until the density has become constant,maintaining thetemperature of the solution, throughout the process', by

means of a thermostat at between 38 to 40 C.

While the use of tivo steps in the aging process has been suggestedherein, it will be appreciated that it is not intended to place anyrestriction upon the process other than those Yset forth in theclaims,but that the process may be carried out in any desired manner such as,for example, simply stirring water in contact with Rochelle salt until aconstant density is reached.

A stock solution is then prepared by slightly under-saturatiug thesaturated solution of the previous step by the addition of water whosetemperature is not above 40C. and filtering, after which theundersaturat ing process is continued to thev desired degree. It hasbeen found desirable to maintain a container of water in the samethermostat used in the aging process so that water, having the sametemperature as the saturated solution, .may be used; in` this manner,the temperature of the mother liquor. during preparation may be moreaocurately controlled. The use of a slightly under-saturated solution inplace of the usual saturated solution has been found desirable as thetemperature of such a solution may be lowered several degrees below 38C., depending upon the degree of under-saturation, before saturationwith its resultant spontaneous precipitation results. It has been founddesirable to eliminate spontaneous precipitations as those solidparticles which are thereby formed have been found to act as individualnuclei in the crystallization stage, and owing to their presence, anumber of small crystals are formed instead of one large crystal as isdesired. The possibility of allowing a reduction in the temperaturebefore spontaneous precipitation results is desirable in that suchreductions in temperature become unavoidable during the transfer of themother liquor from one stage to another, such as inthe filtering step,or in the transfer of the liquor to crystallizing dishes, for example. Astock solution as prepared above is suitable for use as a mother liquorfor ngrowing crystals at any desire-d time and may be maintained in goodcondition so long as the temperature is always maintained between 38 and40 C.

The final, or crystallization, stage consists in inserting a seedcrystal, of the. type described above, in a quantity of this stocksolution in a crystallizing dish or other suitablev container, andcarrying the tempera ture from 38-400' C. down to approximately .20 C.over a period of from 1() to 12 hours,

causing rapid cooling at the start in order to vinduce growth of thehour-glass structure. This desired temperature gradient may besatisfactorily produced by enclosing the crystallizing dish within'abooth for eX- ample, the booth temperature being constant at about 20 C.By enclosing the liquor within such a. booth, the cooling action of aircurrents is substantially eliminated and the temperature drop of themother liquor is therefore gradual. The employment of a boothtemperature -of about 20 C. insures rapid cooling at `the start as themother liquor, when introduced into the booth, is at a. temperature ofabout 33 C.;

after this rapid cooling, however, the mother liquor temperaturegradually approaches the,VT ln this manner rapid' booth temperature.cooling results at the start, while thereafter, the temperaturedecreases more slowly and the ratio of the ten'lperature drop to thetime becomes smaller as the time increases. rlhese conditions oftemperature history have been found to produce very satisfactoryresults.

`While rapid cooling has been found to be desirable in order to insuregrowth of the hour-glass structure, the cooling should not be too rapid,as it has been found that sudden drops in temperature may often resultin spontaneousprecipitations, as pointed out above.

Applicant has found, in practising methods of growing Rochelle saltcrystals, that satisfactor and uniform results were not obtained i atany time during the history of the solution, the temperature had beenraised above 40 C. Heretofore, it has been common practice to warm themother liquor, during preparation, in order to facilitate solution ofthe Rochelle salt. ln such practice, applicant has found that if thetemperature has at any time during preparation of the solution beenabove 40 C., variations in the crystals formed resulted; thesevariations refer principally to the number and uniforn'iity of crystalsformed in the crystallization stage. In accordance with the features ofthis invention, therefore, the mother liquor should possess a wellregulated temperature history so that the temperature has never beenAabove 40 C. and preferably has always been at 38 C.

In a method of crystal production Vwhich applicant has foundsatisfactory, Rochelle salt is allowed to stand in contact with waterfor a period of several months, the temperature always being maintainedbelow 40 C. This solution is vthen stirred at 37.5 to 39.5 C. in contactwith Rochelle salt until the density hasbecome constant at a valueapproximately 1.39,(38o C./50 G). Water, whose temperature has likewisebeen controlled at approximately 38 C., is added, with stirring, to theconstant .density solution until its density reaches a' value between1.37 (38 C./5o C.) and 1.38 (38 C./59 0.). This solution is .thenfiltered and after being filtered is maintained at a temperature ofintocrystallizing dishes, which in turn, are placed in a booth, such asdescribed above, and a seedling inserted, as likewise pointed out above.Crystals grown in this manner have been found to possess the propertiesand characteristics desired forpiezo-eleztric appliances, `and further,the results obtained have given indications yof great uniformity.

'lihe use ofthev figures 38 C./50 C. herein in connection with densityreadings is intended to define the density of a substance at atemperature of 38 C. as given by a hydrometer which is calibrated foruse at 50 C. ln other words, it specifies the relation in weight betweenequal volumes of a particular substance and water, the temperature ofthe substance being 38 U. and that of the water 50 C.

The amount of water added to under-saturate the mother'liquor andtherefore, also, the density of the mother liquor will depend primarilyupon the degree of undersaturation which is desired. rl`his degree ofunder-saturation will, of course, depend upon the amount of coolingwhich it is estimated the liquor must undergo before actualcrystallization is desired. lThe density of the unsaturated motherliquor may be varied, for example, between 1.36 and 1.39 (38 C./50 C.)This variation in the degree yof under-saturation employed makes thismethod applica-ble over a wide range of conditions to which previousmethods were not applicable.

ln selecting raw material to produce a mother liquor for use in themethod of the invention it has been found that commercial Rochelle saltregardless of its degree of dehydration or decomposition, may be used,as long as it is free of other impurities. It is desirable, however, topulverize the raw material employed in order to hasten solution, as ithas been noted that an especially long` period of time is required toform a saturated solution of Rochelle salt from dehydrated crystals ofRochelle salt.

lt has also been found desirable in many instances to cover the surfaceof the liquor during the crystallization stage in some suitable ma-nner,so that the surface will not be cooled appreciably more rapidly than theremainder of the liquor. A porous covering stretched across the top ofthe crystallization dish, forinstance, would retard such cooling.yAnother method of retar-ding this surface cooling action might be tocover thesurface of the liquor with a warm metallic plate. y

In the elimination of spontaneous precipitations, it has'been founddesirable not only to prevent chilling, as noted above, but also toprevent the subjection of the mother lliquor to vibrations during thecrystallization stage, as it has been noted that spontaneousprecipitations are often caused by villll brations. 'It has further beennoted in this respect that the mother liquor should not be contaminatedby any small particles of foreign matter as such particles act, in manyinstances, similar to spontaneous precipitations, as nuclei for crystalgrowth.

cleus from which to grow-a crystal, a fragment of a full grown crystalhaving thedesired characteristics may be used as a nucleus to startcrystallization.- This avoids having to select a seedling crystal of theproper characteristics according to the 'pro` cedure pointed out above,and gives a 'greater assurance of obtaining a crystal having the desiredcharacteristics.

What is claimed is: l

1. The method of preparing a mother liquor from which to growhour-glass. crystals of Rochelle salt, comprising dissolving Rochellesalt in water to secure complete equilibrium between the solvent and thesolute, maintaining the solution at all times at a temperature notexceeding C. and adjusting its density to a value between 1.37 and 1.38(38 C./50 0.).

2. The method of producing Rochelle salt crystals comprising dissolvingRochelle salt in water, aging the resulting solution at a temperature atleast as low as 40 C., undersaturating said solution by adjusting thedensity to a-value between 1.36 and 1.39, and growing the crystals fromsaid solution by applying rapid cooling.

3. The method of producing Rochelle salt crystals comprising forming lasolution of Rochelle salt in water, maintaining said s0- lution incontact with Rochelle salt between and Instead of using a small crystalas a nu.

, urating said solution still maintaining the temperature within thementioned region, owing crystals from 4said solution by startlng rapidcooling from said temperature region.

4. In the.' art of growing Rochelle salt crystals from a solution of thesalt in4 water,

the method comprising maintainingthe solution from which the crystalsare grown at a tem erature atleast as low as 40 C.V

from an vduring thetime the solution is made until the crystal is grown.

vT he method of producing crystals of Rochelle salt which comprisesproducing a mother liquor and growing crystals from said liquor, at alltimes maintainm the temperature of said liquor below 40 6. The method ofproducing crystals of` Rochelle salt having the hour-glass structurewhich comprises producing a nearly saturated solution of Rochelle salt,vplacing a nucleus composed of Rochelle salt in contact with saidsolution, controllably regulating the temperature of `said solution sothat its temperature has never been above 40 C., and lowering thetemperature of sai-d solution after insertion of said nucleus, at a rateadapted to cause a degree of supersaturation.

favorable for growth of the hour-glass struc# ture.

In witness whereof, I hereunto subscribe my name this 29th day ofSeptember A. D.,

ALEXANDERA M. NioonsoN.

